There are many different types of espresso drinks, including cappuccino, cafe latte and cafe mocha. All are made with one or more shots of espresso. A shot of espresso is made by forcing about 1.5 ounces of hot water through tightly packed, finely ground espresso coffee. There are many variables in the process of making a shot of espresso. The temperature of the water, the pressure of the water, the fineness of the ground coffee and how tightly the coffee is packed to name a few. To force the water through the coffee, the simplest espresso machines use pressure that comes from heating water inside a sealed vessel. In this type of machine, the coffee is packed into a funnel-shaped piece of metal that has a tube extending to the bottom of the reservoir. A few ounces of water are put into the reservoir and the top is screwed on.
When the water is heated, pressure builds inside the vessel, and the only way for it to escape is up the tube, through the coffee and out of the tube in the top. Since the end of the tube is under water, the pressure forces the hot water up through the tube. To begin the operation, the brew process is started once the heating vessel has heated the water to the ideal temperature (just below boiling). Ground espresso coffee is poured into the basket and tamped down, and then the porta-filter is installed by twisting it into the machine. A small cup is placed beneath the spout, and the valve to the espresso position is opened. The opening of the valve engages the micro-switch that starts a pump, which in turn pressurizes the heating chamber and hot water to about 15 atmospheres (220 psi) of pressure. This forces the hot water through the ground coffee and out of the spouts. Ideally, it should take about 25 seconds for about 1.5 ounces of espresso to come out.
The problem with espresso machines of this type is that the excess buildup of pressure must be relieved after the operation is complete. The residual pressure is relieved through the same valve and spout as the coffee exits. The operator has the potential exposure to the espresso vessel bursting upon the release of the brew head interface. The residual heat and steam causes the potential pressure through the same brew/froth ports, if there is no other means of eliminating the pressure.